Add a variant to enable libev usage in `scripts/build/builders/host.py`

[plan44]

Could you add a variant to enable libev usage in scripts/build/builders/host.py ?
That would make it easier to build and test this and potentially also use it in our CI builds if we need to maintain its compilation ability.

Originally posted by @andy31415 in #24232 (comment)

[plan44]

I'd need some guidance/hints how to do that.

I managed to understand the single path through the gn build needed to use chip as a SDK submodule from an external libev based Linux daemon project, but am not familiar enough yet with all the build and CI automation to see what exactly is requested here.

It is not possible to just enable CHIP_SYSTEM_CONFIG_USE_LIBEV and compile an existing example that way - the libev based main app would be missing.